Well you're not lifting it straight up in the air!
Pull might be 800 lbs in most cases.
Problem is winches are not rated for dead lift. They are rated for rolling line weight. What you're thinking of isn't a winch rather a hoist. Different animal.
And a Jeep stuck to its belly pan in gooey mud will ACT like 12,000 pounds when the winch has to not only pull the Jeep THROUGH the mud (pushing close to double the weight of the Jeep in mud out of the way in the process), but most people try to get up on TOP of the mud on the way.
So yes, it IS lifting it. And not straight up. It's pulling the Jeep up on top by pulling it sideways, which can increase the weight of the Jeep that the winch sees by several multiples by way of leverage (against the winch).
In the picture below, the winch is pulling along the red axis. The Jeep needs to move in the green axis. So for every inch the winch has to pull forward, it also has to impart 3 inches upwards, which you'd have to ask an engineering major what that works out to be, but I would imagine it multiplies the Jeep's weight by four.
This also applies to going over a rock or a log (anything that you have to climb over to move forward), which is why even with the double torque multiplier of 4 Lo, sometimes you can't get over an obstacle.